PRINCIPAL IDENTIFYING FEATURES OF THE SYNDROME OF NONVERBAL LEARNING DISABILITIES
IN CHILDREN

by Byron Rourke

The identifying features of the syndrome of nonverbal learning disabilities
(NLD) were examined with a view to determining their relative discriminant validity. A
stepwise linear discriminant function analysis of children with NLD (n=29), children with
reading and spelling disabilities (Group R-S; n=27), and a group of nonclinical children
(NC; n=27) on 15 neuropsychological variables yielded a subset of scores on four tests
(Target Test; Trail Making Test, Part B; Tactual Performance Test; and Grooved Pegboard
Test) that accurately (>95%) discriminated the NLD groupfrom the R-S and NC subjects.
Of the neuropsychological features of NLDdescribed by Rourke (1987, 1988b, 1989), deficits
in visua-perceptual-organizational psychomotor coordination and complex tactile-perceptual
skills appeared to be most representative (in the sense of most discriminative) of the NLD
syndrome in the children examined. These are also the dimensions that are considered to be
"primary" in the NLD model (Rourke, 1989). Replication of these results,
employing children with other clinical disorders, is necessary.

Since 1971, we have intensively investigated two subtypes of children
withlearning disabilities (LD). Children in one group (referred to as Group R-S) are those
who exhibit many relatively poor psycholinguistic skills in conjunction with very
well-developed abilities in visual-spatial-organizational, tactile-perceptual,
psychomotor, and nonverbal problem-solving areas. They exhibit very poor reading and
spelling skills but significantly better, though still impaired, mechanical arithmetic
competence. The other group--which we refer to as having the nonverbal learning
disabilities (NLD) syndrome--exhibits outstanding problems in
visual-spatial-organizational, tactile-perceptual, psychomotor and nonverbal
problem-solving skills within a context of clear strengths in psycholinguistic skills,
such as rote verbal learning, regular phoneme-grapheme matching, amount of verbal output,
and verbal classification. Children with NLD experience their major academic learning
difficulties in mechanical arithmetic, while exhibiting advanced levels of word
recognition and spelling (see Note). Both of these subtypes of children with LD have been
the subject of much scrutiny in our laboratory (for reviews, see Rourke, 1975, 1978, 1982,
1987, 1988a, 1988b, 1989, 1993; Rourke & Finlayson, 1978; Rourke & Fisk, 1988,
1992; Rourke & Fuerst, 1992; Rourke & Strang, 1978, 1983; Strang & Rourke,
1983, 1985a, 1985b).

Nonverbal Learning Disabilities

The NLD syndrome has its research roots in studies that have been
conductedin our laboratory since the late 1960s, some conclusions of which have
beenoutlined above. The principal clinical manifestations of the NLD syndrome are as
follows:

1. Bilateral tactile-perceptual deficits, usually more marked on the left side
of the body;

2. Bilateral psychomotor coordination deficiencies, often more marked on the
left side of the body;

4. Marked deficits in the areas of nonverbal problem solving, concept formation,
hypothesis testing, and the capacity to benefit from positive and negative informational
feedback in novel or otherwise complex situations. Included are significant difficulties
in dealing with cause-effect relationships and marked deficiencies in the appreciation of
incongruities (e.g., age-appropriate sensitivity to humor);

8. Much verbosity of a repetitive, straightforward rote nature. Content
disorders of language, characterized by very poor psycholinguistic pragmatics (e.g.,
"cocktail party" speech). Misspellings almost exclusively of the
phonetically accurate variety. Little or no speech prosody. Reliance on language as a
principal means for social relating, information gathering, and relief from anxiety; and

9. Significant deficits in social perception, social judgment, and social
interaction skills. A marked tendency toward social withdrawal and even social isolation
as age increases. (Such children are very much at risk for the development of
socioemotional disturbance, especially "internalized" forms of psychopathology.)

We have found that the NLD syndrome is manifested most clearly on
a "developmental" basis. However, it is also seen in persons suffering from
a wide
variety of neurological diseases and disorders. These include significant tissue
destruction within the right cerebral hemisphere, some types of hydrocephalus, many types
of head injury, and other neuropathological processes that have as one of their results
significant destruction of neuronal white matter (long myelinated fibers). In addition to
describing the clinical features of this syndrome, a model to explain the syndrome's
dynamics has been proposed (Rourke, 1987, 1988b, 1989). The model involves an extension of
the theoretical tenets of Goldberg and Costa (1981), some integration with Piagetian
developmental theory, and some relationships to known age-related developmental changes in
neuropsychological test performance. For a full description of the syndrome and the
"white matter" model designed to account for it, the interested reader is
referred to Rourke (1989).

The principal dimensions of the NLD syndrome are thought to be deficits in
visual-perceptual-organizational abilities, complex psychomotor skills, tactile
perception, and nonverbal problem-solving skills, with age-appropriate development of rote
verbal, simple motor, and psycholinguistic skills and abilities (Rourke, 1989). In the
discussion that follows, it would be well to refer to Figure 1. For our present purposes,
it should be emphasized that we see the patterns of academic and psychosocial deficits
experienced by individuals who exhibit this subtype of learning disability as the direct
result of the interaction of the primary, secondary, tertiary, and linguistic
neuropsychological assets and deficits that are outlined in Figure 1.

For example, considering the hypothesized "deficit" stream, we would
view the primary neuropsychological deficits experienced by the child with NLD as having
to do with aspects of tactile and visual perception, complex psychomotor skills, and the
capacity to deal adaptively with novel material. Such deficits would be expected to
eventuate in disordered tactile and visual attention and stunted exploratory behavior; in
turn, problems in memory for material delivered through the tactile and visual modalities,
as well as deficits in concept formation and problem solving, would be expected to ensue.
This set of deficits would be expected to result in the particular set of linguistic
deficiencies outlined in Figure 1. The academic and psychosocial/adaptive deficiencies
listed are the expected sequelae of these neuropsychological deficits. It is particularly
important to note that we would expect this set of neuropsychological deficiencies to
lead, in a necessary way, to a particular configuration of problems in
psychosocial/adaptive behavior, both within and without the academic situation (Rourke,
1988a, 1989; Rourke & Fuerst, 1992). As suggested above, the reader may find that
periodic reference to Figure 1 will be of some assistance in determining our rationale for
the current study.

Some other researchers who have pursued research along lines similar to our own
(e.g., Grace & Malloy, 1992; Tranel, Hall, Olson, & Tranel, 1987; Voeller, 1986;
Weintraub & Mesulam, 1983) have arrived at conclusions regarding particular salient
dimensions of LD that resemble the NLD syndrome. Some have accentuated the
"right-hemisphere" nature of the deficits, others the psychosocial dimensions,
and still others dimensions such as deficits in prosody. These formulations do not deal
with issues regarding the relative importance of these dimensions. As noted above (and see
Figure 1), the Rourke (1987, 1988b, 1989) formulation of the NLD syndrome and model is
explicit with respect to the dimensions of NLD that are thought to be causative and
sequential (i.e., primary --> secondary --> tertiary --> linguistic) and
dependent (i.e., academic and psychosocial).

At this juncture in our research program, we felt that it would be important to
determine the relative discriminative validity of these various dimensions of the NLD
syndrome. As a first step in this process, we thought it worthwhile to examine the
relative discriminative validity of a wide range of neuropsychological assets and
deficits, as measured by a comprehensive battery of neuropsychological tests. If a set of
discriminators consistent with the Rourke (1989) model were to be found, it would then
make sense to go on to investigate the other dimensions of the model in a similar manner.

Also, from a clinical point of view, although the neuropsychological, verbal,
academic, and psychosocial/adaptive features of NLD have been identified, clearly defined
clinical criteria for use in "diagnosing" this syndrome have not yet been
established. The features that most readily identify the syndrome in children, and their
specificity to NLD, are not known. We felt that this was, in and of itself, a sufficient
reason to attempt to determine the most salient dimensions of NLD.Hence, the purpose of
the present study was to derive a constellation of the features that would be most useful
for identifying children who exhibit NLD.

This was accomplished by examining the relative discriminating power of
theneuropsychological and academic characteristics of the syndrome. The accuracywith which
this constellation of features differentiates children who display NLD from one other
subtype of LD and from children who do not exhibit aclinical disorder provides a test of
the discriminative validity of these selected dimensions of the syndrome.

The distinctive pattern of neuropsychological and academic assets anddeficits
exhibited by children of the R-S subtype described by Rourke and colleagues (Rourke &
Finlayson, 1978; Rourke & Strang, 1978; Strang & Rourke,1983) renders them well
suited for clinical comparison with individuals identified as having NLD. Group R-S
children display deficiencies primarily on tests of a verbal and linguistic/
psychoacoustic nature (primarily those that involve phonemic discrimination, blending, and
segmentation), and on tests involving a large verbal or symbolic component (e.g., some
aspects of mechanical arithmetic, finger graphesthesia). However, R-S children have been
shown to exhibit age-appropriate development of visual-perceptual- organizational
abilities, psychomotor coordination, complex tactile-perceptual abilities, and
concept-formation and problem-solving abilities (Rourke & Finlayson, 1978; Rourke
& Strang, 1978; Strang & Rourke, 1983). The R-S subtype is most similar to the
phonological-processing subtype of disabled readers, which has been investigated
intensively by Shankweiler and Liberman (1989), Stanovich (1988), and Torgesen (Wagner
& Torgesen, 1987). (The interested reader may wish to consult Torgesen, 1993, for
asystematic comparison of this phonological subtype of disabled reader and theNLD subtype
[syndrome].)

Incorporating these two clinical groups (NLD and R-S) and a third,nonclinical
(NC) group in this study allowed for the framing of some specific hypotheses with respect
to the relative discriminative accuracy of various measures and dimensions vis-a-vis NLD,
as follows:

1. The NLD group was expected to perform worse than--and, thus, be
distinguishable from--the NC group on tests sensitive to those skills that have been found
to be deficient in children with the NLD syndrome (e.g., visual-perceptual-organizational,
psychomotor, tactile-perceptual, mechanical arithmetic, and conceptual and
problem-solving).

2. Children within the R-S group were expected to exhibit age-appropriate
development of these skills and abilities (with the exception of mechanical arithmetic);
in addition, they were expected to be distinguishable from the NLD group by their
relatively better performance on tests within the aforementioned realms.

3. The NLD group was expected to perform better than the R-S group on measures
of some (primarily rote and straightforward) verbal and psycholinguistic abilities that
are thought to develop in an age-appropriate manner in individuals who exhibit NLD, but
not in children in the R-S group.

4. It was expected that the performances of the NLD and NC groups would not be
distinguishable on the aforementioned verbal and psycholinguistic measures.

5. It was expected that those dimensions thought to be primary in the NLD
syndrome (i.e., visual-spatial-organizational, tactile-perceptual, psychomotor) would be
the principal variables that distinguished the NLD group from the R-S and NCgroups.

Method

Subjects

The data used in the present study were drawn from two sources. A total of 29
children who exhibited the NLD syndrome and 29 children who matched the R-S academic
achievement test profile were selected from a clinical data base ofover 5,000 children who
had received neuropsychological assessment because of suspected learning or perceptual
difficulties. Data representing 25 of the 29 NLD subjects were collected as part of an
earlier study (Casey, Rourke, & Picard, 1991). The data for 4 additional subjects were
added to these 25 cases. Twenty-nine subjects free of learning or perceptual difficulties
served as the nonclinical comparison group and were selected from data collected as part
of a longitudinal study of children with reading disabilities (Rourke & Orr, 1977).

None of the 87 subjects used in the present study exhibited primary
socioemotional disturbance, sensory impairment, socioeconomic deprivation, or educational
disadvantage. All spoke English as their native language. Subjects
were matched for age; no significant differences between the three groups
were evident, F(2,80)=1.66, p > .10. In accordance with the procedures used in
Casey et al. (1991), a two-step process was employed to select the NLD subjects. First, a
broad-based set of neuropsychological test criteria was employed to search the clinical
data base for all children who exhibited the following constellation of characteristics:
(a) good verbal capacity, (b) difficulty with mechanical arithmetic, (c)
visual-perceptual-organizational deficiencies, (d) psychomotor deficits, and (e)
tactile-perceptual deficits. The specific test criteria for these characteristics are
provided in Table 1. Forty-two cases were identified in this manner.

At this point we would do well to emphasize that we are concerned in this study with the
relative discriminative accuracy of various dimensions that are thought to be typical of
children with NLD. That we chose children in terms of dimensions that are thought to be
typical of such children (see above) is of no consequence. What is important to realize is
that, having chosen children in terms of these criteria, we were then in a position to
determine the relative discriminative (i.e., concurrent validity) power of
these several and various dimensions. The second step in the selection process involved a
review of each case by two clinicians to determine if the child in question exhibited the
behavioral consequences of the NLD syndrome. Twenty-nine cases were identified, and the
reviewers were in 100% agreement that each case was consistent with the expected
behavioral expression of NLD. Of the remaining cases, eight were
dropped because they (a) exhibited features not consistent with NLD or (b)
demonstrated difficulties at the time of testing that rendered their results unreliable.
Five additional subjects were excluded because they were too young and could not be
matched for age with subjects in the other groups. Following the procedures employed in
earlier studies of children with reading and spelling disabilities (e.g., Rourke &
Finlayson, 1978), the R-S subjects were selected solely on the basis of their pattern of
performance on the Wide Range Achievement Test (WRAT) (Jastak & Jastak, 1965).
Children whose grade-equivalent scores on the Reading and Spelling subtests were at least
1.8 years below their grade-equivalent score on the Arithmetic subtest, and
whose Reading and Spelling subtest centile scores did not exceed 14, were
included in the R-S group. It is important to note that the R-S group was chosen solely on
the basis of their pattern of performance on the WRAT, and not in terms of their
neuropsychological assets and deficits (see Rourke, 1989, Chapter 8).

Testing and Measures

Subjects were individually administered a battery of neuropsychological tests
(as outlined in Rourke, 1989) by extensively trained psychometric technicians in a
standardized fashion. To guard against spurious classification results due to a small
subject-to-variable ratio (Fletcher, Rice, & Ray, 1978), only 15 neuropsychological
variables were used in the present study. Eight variables were chosen because they
represented skills and abilities identified as deficient within the NLD syndrome (Rourke,
1989); these included (a) the Category Test (Reitan & Davison, 1974); (b) the
Arithmetic subtest from the WRAT; and (c) memory and (d) location scores from the Tactual
Performance Test (TPT) (Reitan & Davison, 1974). Several composite scores were created
to maximize the power of the analysis and to reduce alpha bias. Dominant- and
nondominant-hand scores from the Grooved Pegboard Test (Klove, 1963) and the TPT were
averaged to yield composite (e) Grooved Pegboard and (f) TPT scores.

Dominant-and nondominant-hand scores on the finger agnosia and finger
dysgraphesthesia portions of the Reitan-Klove Sensory-Perceptual Exam (Reitan, 1984) were
averaged into a single (g) Tactile score. Finally, performances on the Target Test
(Reitan, 1966) and the Trail Making Test, Part B (Reitan & Davison, 1974) were
collapsed to form (h) a Visual-Perceptual-Organizational measure. Seven tests on which
children who exhibit the R-S academic achievement profile are expected to perform poorly
(Rourke & Finlayson, 1978) were also included: (i) the Peabody Picture Vocabulary Test
(PPVT) (Dunn, 1965); (j) the WRAT Reading subtest; (k) the WRAT Spelling subtest; (1) the
Speech-Sounds Perception Test (SSPT) (Reitan & Davison, 1974); (m) the Auditory
Closure Test (Kass, 1964); (n) the Sentence Memory Test (Benton, 1965); and (o) a
phonemically cued test of verbal fluency (Rourke, Bakker,
Fisk, & Strang, 1983). (Although the R-S subjects were selected on the basis of their
pattern and levels of performance on the WRAT, it was necessary to include these academic
achievement measures in the analysis so that comparisons could be effected with the NLD
and NC groups on these important academic dimensions. In addition, we thought it was
important to determine the relative discriminating power of these academic measures.)
Scores on the selected neuropsychological and academic tests were converted to
age-corrected T scores (M=50, SD=10) to allow for intergroup comparisons.

Conversion of most of the neuropsychological test scores into T scores was based
on the norms of Knights and Norwood (1980); scores for the WRAT subtests and the PPVT were
converted based on their respective norms. T scores were calculated so that larger values
represented better performance.

Results

Table 2 contains the descriptive characteristics for each of the three groups.
Subjects in all groups were predominantly right-handed. All of the NC sample and the
majority of the R-S sample consisted of males; the NLD cases were more equally divided
into males and females--the proportion that is most often found (Rourke, 1989).

Of the original 87 cases, 4 subjects (2 from the R-S group and 2 from the NC
group) were identified as outliers, due to extreme scores on several of the tests, and
were dropped from further analyses. Assumptions of linearity, normality, homogeneity of
the variance-covariance matrices, and multi-collinearity or singularity were evaluated,
based on the remaining 83 cases, and the results revealed no threat to multivariate
analysis (Tabachnick & Fidell, 1983).

A one-way multivariate analysis of variance of the full 15-variable model was
significant at the p<.001 level. Univariate tests of analysis of variance were,
therefore, conducted across each variable. For all 15 variables, differences between
groups were significant at least to the p<.05 level. Table 3 provides the means,
standard deviations, and univariate F statistics for the 15 measures used in this study.

A series of planned comparisons was conducted to test the study hypotheses. The
results of the group comparisons on all but the verbal-fluency variable were significant
at the p<.01 level. The comparison of the three groups on the verbal-fluency variable
was significant at the p<.05 level. As illustrated in Figure 2, the NLD group performed
significantly more poorly than did the R-S and NC groups on the
Visual-Perceptual-Organizational, Grooved Pegboard, Tactile, and TPT composite measures,
and on the WRAT Arithmetic, TPT-Memory, TPT-Location, and Category Test variables.
Furthermore, the performance of the NLD group on all variables except the TPT-Memory and
Category Test variables fell within the impaired range (i.e.,
a T score <.40). In contrast, the performance of the NLD group on the verbal
neuropsychological and academic tests was within the average range. The R-S
group performed more poorly than did the NLD and NC groups on the PPVT, SSPT, Auditory
Closure, Sentence Memory, verbal fluency, and WRAT Reading and Spelling variables.

To evaluate which of the neuropsychological and academic features of the NLD
syndrome would be most useful in distinguishing the NLD cases from the remaining subjects,
a stepwise linear discriminant function analysis was performed. In calculating the linear
discriminant function analysis, test measures were employed as predictors and the analyses
maximized Wilk's lambda. Approximately 75% of the cases were randomly selected to be used
in the initial analysis. The remaining 25% of the cases were reserved for later
cross-validation.

Two significant discriminant functions resulted, combined x[sup 2] (18, n = 63)
= 210.61, p<.001. After removal of the first function, the second discriminant function
retained a high degree of discriminating power, x[sup 2] (8, n=63) = 85.97, p<.001. The
two discriminant functions accounted for 69% and 31% of the total variance, respectively.
The first function discriminated the NLD group from R-S and NC groups; the R-S group was
separated from the NLD and NC subjects by the second discriminant function.

Eight of the 15 variables combined to account for the majority of the power in
discriminating the NLD groups from the R-S and NC groups. The resultant predictor
variables are provided in Table 4, as are their correlations with each of the two
functions and their standardized canonical discriminant function coefficients. Increases
in the number of predictors beyond the eight-variable model failed to increase the
discriminating power of the functions significantly, F>1.0.

As can be seen from Table 4, three measures (Visual-Perceptual-Organizational,
Grooved Pegboard, and TPT) correlated substantially (r's= .45 to .60) with the first
function to discriminate the NLD group from the other two groups. Two measures (SSPT and
WRAT Reading) loaded highly (r's = .44 and .79, respectively) on the second function to
distinguish the R-S cases from the NLD and NC subjects.

Classification of the sample was performed using Lachenbruch's Leaving-One-Out
Method (Fletcher et al., 1978; Lachenbruch, 1975) to reduce bias that would contribute to
overfitting of the data. Using sample proportions as prior probabilities, all but one of
the NLD cases (95%) and all of the R-S and NC cases were correctly classified, with an
overall accuracy of 98%. A classification table is provided as Table 5.

Cross-validation with the hold-out sample resulted in all of the NLD and R-S
subjects being correctly identified. The three misclassifications that did occur in the
cross-validation came from the NC group.

Discussion

The principal finding of this study was that a subset of four neuropsychological
tests (the Target Test; the Trail Making Test, Part B; the Tactual Performance Test; and
the Grooved Pegboard Test) served to discriminate the NLD subjects from the R-S and NC
subjects with a high degree of accuracy (> 95%). Two tests, the Reading subtest of the
WRAT and the Speech-Sounds Perception Test, best discriminated the R-S children from the
NLD and NC children. Several points should be mentioned regarding these findings: First,
of the major (i.e., primary) neuropsychological features of NLD described by Rourke
(1987), deficient age-related development in the realms of
visual-perceptual-organizational ability, psychomotor coordination skill, and complex
tactile-perception ability appear to be most characteristic of the syndrome in children.
It would seem reasonable, therefore, to devote attention to children's performance within
these realms when the clinician is considering making a diagnosis of possible NLD, as well
as when treatment programs are being considered.

Second, in interpreting the present results in terms of the NLD model described
at the beginning of this article (see, also, Rourke, 1989), it becomes apparent that the
neuropsychological deficits that distinguished the NLD group from the other two groups are
those that are thought to be primary in the dynamics of this disorder (see Figure 1). A
recent investigation by Casey et al. (1991) helps to explain why this might be expected.
Casey et al. examined the manner in which the neuropsychological assets and deficits of
children with NLD change over the course of development, by conducting a cross-sectional
comparison study of the performance of NLD subjects in middle
childhood to that of others in their early adolescence across a comprehensive array of
neuropsychological tests. They found that the younger children were most deficient in
spatial-organizational skills, complex motor skills, complex tactile skills, and
problem-solving skills (e.g., see Figure 6 in Casey et al., 1991). Further-more, an
age-related decline in these same functions was more marked than for other skills and
abilities. Thus, deficits that are thought to play a primary role in the manifestation of
the NLD syndrome involve skills that not only are more poorly developed initially, but
also fail to develop to the same extent or at the same rate as do most other
neuropsychological skills and abilities.

Therefore, it is not surprising that the most salient identifying features of
the NLD syndrome in children correspond to those skills and abilities that have been found
to be among the least developed initially, and that continue to worsen (relative to
age-peers) as children with NLD grow older.

Compared to the R-S and NC groups, the NLD group performed more poorly on tests
of visual-perceptual-organizational skills, psychomotor coordination, complex
tactile-perceptual skills, and conceptual and problem-solving skills. Furthermore, the NLD
group's levels of performance were within the normal range and did not significantly
differ from those of the NC group on tests of the more rote aspects of verbal and
psycholinguistic skills. These findings correspond to the pattern of neuropsychological
and academic assets and deficits that has been described for the NLD syndrome (Rourke,
1987, 1988b, 1989).

The R-S group performed more poorly than did the NLD and NC groups on the tests
of rote verbal and psycholinguistic abilities, single-word reading, and spelling skills
employed. Unlike the NLD group, however, the R-S children performed in an age-appropriate
manner on tests of visual-perceptual-organizational skills, psychomotor coordination,
tactile perception, memory for tactile information, and concept-formation and
problem-solving abilities.

Although this study was not designed with this purpose in mind, it is
interesting to note the close resemblance of the NLD and R-S groups' patterns of
neuropsychological assets and deficits, as illustrated in Figure 2, to the findings of
earlier research demonstrating differing patterns of neuropsychological assets and
deficits between children of the R-S subtype and children who exhibited outstanding
difficulties in mechanical arithmetic (Rourke, 1993; Rourke & Finlayson, 1978; Rourke
& Strang, 1978; Strang & Rourke, 1983, 1985b). The results of the present study
are quite similar to those illustrated in Figure 8-1 in Strang and Rourke (1985b); see
also Rourke
(1993, Figure 1). The NLD group's general level and pattern of performance on
measures of visual-perceptual-organizational ability, psychomotor coordination,
tactile-perceptual ability, and conceptual and problem-solving ability in the current
study are comparable to those found for children who exhibit outstanding difficulties in
mechanical arithmetic and average to above-average single-word reading and spelling
skills. Similarities in patterns of performance on tests of verbal and psycholinguistic
skills are also evident, although the children with NLD were less proficient on these
tests.

Finally, the R-S group's performance fell below age expectations on two psycholinguistic
tests (the SSPT and the Auditory Closure Test), and these children performed worse than
the NLD group on all verbal and psycholinguistic tests, except for the phonemically cued
test of verbal fluency. Thus, the present findings lend confirmatory support to the
results of earlier subtyping studies (e.g., Rourke & Finlayson, 1978; Rourke &
Strang, 1978; Strang & Rourke, 1983) that formed the basis for our current
understanding of the NLD syndrome.

Limitations of the Study

In interpreting the results of the current investigations, three caveats should
be noted. First, caution is advised in interpreting the Target Test; the Trail Making
Test, Part B; the Grooved Pegboard Test; and the Tactual Performance Test as being the
best neuropsychological predictors of NLD in children. Although the present study yielded
a subset of these four tests that best discriminated the children with NLD from those with
R-S and NC children, other combinations of tests or other individual tests (e.g., the
Category Test) may have enough predictive utility to warrant their consideration in any
number of circumstances.

This situation arises due to the nature of the stepwise discriminant analysis.
Predictor variables are selected for entry into the discriminant function equation on the
basis of the size of their correlations with the function, and the amount of
"independent" variance (i.e., variance not shared with other predictor
variables) that they have to offer. Variance that is shared among intercorrelated
predictor variables is assigned to the predictor that has the greater relationship with
the discriminant function.

Consequently, following the entry of one of a pair of intercorrelated predictor
variables into the discriminant function equation and removal of the variance it shares
with the other variable, the remaining predictor may lack sufficient independent variance
to satisfy minimal entry requirements (Fletcher et al., 1978).

Second, the nature of the clinical group selected to serve as a contrast to the
NLD sample probably contributed to the highly accurate rate of classification. High rates
of correct identification are expected when groups exhibiting extreme differences on the
predictor variables of interest are employed in a linear discriminant function analysis
(Adams, 1979). Previous investigations employing samples of children of the R-S subtype
who were selected according to criteria similar to those used in the present study (Rourke
& Finlayson, 1978; Rourke & Strang, 1978; Strang & Rourke, 1983) have
demonstrated that these children exhibit neuropsychological test performances that are
quite different and distinct from those that are expected for the NLD sample (e.g.,
Fletcher, 1985). Consequently, the rates at which membership for each group was correctly
predicted were expected to be greater than would be the case if less-differentiated groups
(e.g., the "Output Disorder" subtype: Rourke, 1989, Chapter 8) were included in
the analysis.

Third, linear discriminant function analysis co ntributes to inflated accuracy
of classification through the minimization of the amount of variance not attributable to
between-group differences (Adams, 1979). The concern over possible overfitting of data is
accentuated when stepwise procedures are employed (Tabachnick & Fidell, 1983).
Although methods of classification were adopted that minimized spurious accuracy rates,
inflated classification cannot be ruled out in the results of the present study.

Although the results of this investigation serve as an initial step in
developing reliable criteria for identifying the NLD syndrome in children, several
important aspects still need to be addressed. An insufficient amount of research has
focused on deriving constellations of neuropsychological assets and deficits that would
distinguish children who display the NLD syndrome from children displaying other clinical
disorders. Replications of this study, employing clinical groups other than an R-S LD
subtype, could address this issue and would contribute to the establishment of a critical
set of clinical features for diagnostic purposes.

Finally, Rourke (1987, 1988b, 1989) suggested that children who have experienced
clinical conditions in which substantial damage to cerebral white matter resulted (e.g.,
early-acquired moderate-to-severe closed-head injury) provide profiles of
neuropsychological, academic, and socioemotional functioning that closely resemble those
associated with the NLD syndrome. For example, based on the results of the research
program conducted by Fletcher and associates (e.g., Fletcher & Levin, 1988), the NLD
profile should be most evident in children with severe head injuries (who can be tested)
and less evident in those with mild or moderate injuries.

The nature of the deficits in children with NLD is hypothesized to be radically
dependent upon the severity of white-matter involvement and the age at which the damage
was sustained. Thus, for example, severe or widespread white-matter disturbance occurring
very early in life would be expected to eventuate in deficiencies in virtually all
skills and abilities. However, milder degrees and extents of dysfunction, such as may
occur in later life, would be expected to eventuate in a milder impact on a child's
neuropsychological abilities. In the case of linguistic skills, for example, once these
functions have been reasonably well developed (e.g., by 5 years of age) they tend to be
more impervious to white-matter disturbance. Another severity dimension of relevance in
the psycholinguistic realm is the degree to which the verbal task is novel (Rourke, 1989).

Further comparisons of children with the NLD syndrome to those who exhibit other
clinical and/or medical conditions will aid in the identification of childhood clinical
disorders that have commonality with the NLD syndrome. This should help to further
elucidate the mechanisms that underlie the NLD syndrome and provide opportunities to
establish firmer diagnostic criteriafor it. Recent attempts to do this have proven to be
quite fruitful (e.g., Ewing-Cobbs, Fletcher, Levin, & Boudousquie, 1993; Fletcher et
al., 1992; Rovet, 1993; Sparrow, 1993; White, 1993).

We gratefully acknowledge the assistance of Joseph E. Casey and Darren R.Fuerst, who commented on previous drafts of this article.

NOTE:

Children with "specific" arithmetic disabilities are referred
to as "Group 3" in Rourke and Finlayson (1978), Rourke and Strang (1978), and
Strang and Rourke (1983), and as "Group A" in Strang and Rourke (1985a, 1985b).

Reitan, R. M. (1966). A research program on the psychological effects of
brain lesions in human beings. In N. R. Ellis (Ed.), International review of research in
mental retardation (pp. 153-218). New York: Academic Press.

Michael C. S. Harnadek completed his PhD at the University of Windsor and
is
now with the Department of Psychological Services at University Hospital,
London, Ontario. Byron P. Rourke, PhD, is professor of psychology at the
University of Windsor. Past president of the International Neuropsychological
Society and of the Division of Clinical Neuropsychology of the American
Psychological Association, he is currently president of the American Board of
Clinical Neuropsychology. He is cofounder and co-editor of the Journal of
Clinical and Experimental Neuropsychology, The Clinical Neuropsychologist,
and The Journal of Child Neuropsychology. Address: Byron P. Rourke,
Department of Psychology, University of Windsor, Windsor, Ontario, Canada N9B
3P4.